MRI methods and apparatus of numerous commercially available designs are by now well known. MRI utilizes nuclear magnetic resonance (NMR) phenomena to provide displayed images of internal patient anatomy. The anatomy to be imaged is subjected to a strong static magnetic field to generally align therewith the magnetic moments of a significant number of one or more nuclear species (e.g., hydrogen atoms in water molecules). An RF (radio frequency) magnetic field (at the NMR Larmor frequency) is radiated into the anatomy to be imaged so as to nutate these aligned magnetic moments from the static field by a desired amount. Then, the initial nutating RF field is turned off and a characteristic RF NMR response is then generated as the nutated nuclei move back toward alignment with the static field. A sequence of controlled pulses of magnetic gradient fields (i.e., still aligned in direction with the static field but now having respective magnitudes that exhibit separate and controllable spatial gradients along each of three orthogonal coordinate directions) and/or further RF nutation pulses are applied to the subject anatomy to elicit detectable NMR RF responses from these nuclei that now have encoded spatial information (e.g., vis-a-vis relative magnitudes, frequencies and/or phases of the detected RF NMR response signals). The detected/decoded RF NMR time-domain responses are then used to populate k-space, the k-space data then being subjected to multi-dimensional Fourier Transformation (FT) so as to generate spatial domain image data for output (e.g., to a display, to storage, to a remote location for display or storage, etc).
However if the patient anatomy being imaged moves during the data acquisition period, motion artifacts are generated in the resulting displayed image. Such motion artifacts may cause difficulties of various kinds if the image is used for medical diagnosis purposes.
One prior approach for reducing motion artifact (e.g., correcting for such) is commonly known as PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction)—also sometimes called the BLADE method. Sometimes hereinafter such technique may be referred to as a “propeller” or “blade” or “propeller/blade” and/or a “blade rotation” data acquisition method. FIG. 17 and FIG. 18 illustrate some aspects of such prior art propeller/blade techniques.
As FIG. 17 indicates, the blade rotation data acquisition method acquires NMR signal data using non-Cartesian grid filling into k-space. This is achieved by rotating a belt-like or blade-like data acquisition region (herein called a “blade”) formed by a plurality of parallel data acquisition loci for which NMR data is acquired at repeated time intervals. Such blade rotation data acquisition can be performed using many different basic and well known NMR data acquisition techniques. For example, one can use the well known FSE (Fast Spin Echo) method which can employ multi shot MRI techniques. Once NMR data for a given position of the blade has been acquired, the blade angle can be rotated around the origin the k-space (e.g., by altering the relative magnitudes of simultaneously applied orthogonal gradient magnetic fields to create a vector sum gradient field for use in further periodically repeated NMR data acquisition sequences within the now rotated blade position). In such blade rotation data acquisition methods, the direction along the longer side of the blade area is used as the read-out (RO) direction and the direction along the shorter side of the blade is used as the phase encode (PE) direction as shown by FIG. 17.
As FIG. 17 indicates, data near the origin of k-space (i.e., low spatial frequency data) exists on every blade. Therefore, comparison between different time-spaced images made by Fourier Transformation of acquired k-space data on respectively corresponding blades enables determination of possible motion-induced displacement of common imaged elements between the repeatedly imaged low spatial frequency region near the origin of k-space.
Then, on the basis of such determined image element displacements, if any, discrepancies between the repeatedly imaged elements (albeit at different respective times) are corrected by well known image rotation and/or translation techniques to produce (and by inverse multi-dimensional FT to go from the spatial domain back to k-space and produce a correspondingly corrected k-space data set) MR images for display (now again using multi-dimensional FT to go from k-space to the spatial domain) having suppressed motion artifact.
As noted, it is common in the blade rotation data acquisition method to use the direction along the longer side of the blade as the read-out (RO). However there are some cases where this blade rotation data correction process can undesirably increase phase errors caused by a non-uniform static magnetic field. To help alleviate this possible problem, another species of blade data acquisition method (Short Axis PROPELLER) can be used. As shown in FIG. 18, an EPI (Echo Planar Imaging) method can be used wherein the direction along the longer side of the blade is used as the phase encode (PE) direction while the direction along the shorter side of the blade is used as the read-out (RO) direction. Of course in this method the blade region is still rotated after repeated time intervals.